1. Field of the Invention
This invention relates to physical quantity sensors, and particular to a physical quantity sensor having a diagnosis function, and more particular to a physical quantity sensor for detecting physical quantity such as pressure and having a diagnosis function.
2. Related Art
A related art sensor apparatus, which is used for detecting pressure in a vehicle such as brake fluid pressure or fuel pressure, will be explained with reference to FIG. 10. This kind of sensor apparatus is provided with a sensor portion 20 for outputting signals depending on pressure applied thereon and a control portion 10 such as an ECU for performing several controls based on the output signal from the sensor portion 20.
A battery power supply Vcc (e.g., 12V) is converted to a constant voltage (e.g., 5V) by a regulator 11 provided in the control portion 10. After being converted, the battery power supply is supplied to the sensor portion 20 via a power supply line LP. The sensor portion 20 performs several functions including a pressure detection, an output amplify, and an adjusting the output by using the supplied constant voltage as power. The sensor portion 20 also outputs output voltage Vo in proportion to applied pressure through an output line LO. The control portion 10 performs several controls based on the output voltage Vo.
It is necessary to electrically connect between the control portion 10 and the sensor portion 20 via the power supply line LP, the output line LO and a ground line LG. Generally, the control portion 10 and the sensor portion 20 are electrically connected by using a connector, soldering, welding or the like. Here, in FIG. 10, terminals of the control portion 10 which connecting each line LP, LO, LG are described as PE, OE, GE; and terminals of the sensor portion 20 which connecting each line LP, LO, LG are described as PS, OS, GS.
However, a poor connection (failure in connection) at a connection point between the control portion 10 and the sensor portion 20 may cause a fault due to an increase of a resistance of the line. Particularly, when such a fault occurs on the output line LO, an additional resistor Rx is added to the connection point. In this case, the output voltage Vo will fluctuate by xcex94Vo (=Ioxc3x97Rx) depending on a current Io in the output line LO. Therefore, when the output voltage Vo from the sensor portion 20 changes, it is difficult to distinguish whether this change is caused due to pressure changes or due to poor connection.
Here, it assumes that a fault due to the poor connection is detected by measuring the output voltage Vo while no pressure is applied to the sensor portion. For example, when a load resistor R having 330 kxcexa9 is provided between the output line LO and the ground line LG, and the output voltage while no pressure is applied to the sensor portion is Vo=0.5 V, the current Io in the output line LO is Io=1.5 xcexcA. Here, a standard output voltage Vo is set to, for example, 0.1 V. A diagnosis system is constructed to detect the fault of the pressure sensor when the output voltage excesses a range of Vo=0.5xc2x10.1 V. In this case, when the resistance is increased by Rx due to the poor connection or the like, the diagnosis system cannot detect the increase of the resistance if the resistance increases by Rx=66 kxcexa9 (=0.1 Vxc3x971.5 xcexcA) or less. Here, the tolerance standard xc2x10.1 V is determined by considering manufacturing deviation of the pressure sensor, and it is difficult to reduce this standard because it causes decrease of productivity of the sensors and it causes increase of manufacturing cost.
Incidentally, in general, the control portion 10 is provided with a load resistor RA between the output line LO and the ground line LG, so that the sensor output (normally within a range of 0.5 Vxe2x88x924.5 V) is positively changed into a fault signal (outside of the normal range, e.g., 4.8 V or more, or 0.2 V or less) when a short-circuit or a cut-off occurs on the lines LP, LO and LG. Therefore, the sensor output VOE to be received by the control portion 10 is determined by a resistance division between the load resistor RA and the increased resistance Rx. In detail, the VO, is calculated by VOE=VOSxc3x97RA/(RA +Rx).
For example, when the load resistor is PL=100 kxcexa9 and the sensor output from the sensor portion is VOS=0.5 V, as shown in FIG. 11, the sensor output VOE received by the control portion 10 changes in response to the resistance Rx added to the output line LO, and the sensor output deviates from the VOS which is outputted from the sensor portion 20. Therefore, even when the sensor portion 20 outputs accurate sensor signal VOS in response to the applied pressure, the control portion 10 performs several controls based on the sensor signal VOE which is different from the sensor signal VOS outputted from the sensor portion 20.
This invention has been conceived in view of the background thus far described and its object is to accurately detect a fault due to poor connection or the like at a connection portion between a sensor portion and a control portion.
According to a first aspect of the present invention, a diagnosis portion measures current flowing in one of external wirings to detect a resistance of a circuit system including the one of the external wirings in which the current flows. Then, the diagnosis portion detects a fault of the circuit system by processing measured current. Therefore, when the resistance is changed as a result of the fault such as a poor connection, such the fault can be easily detected.
According to a second aspect of the present invention, a diagnosis portion measures current flowing from the output line to the ground line. Then, the diagnosis portion detects an occurrence of fault by comparing a measured current value and a predetermined reference current value. Therefore, when the resistance is changed as a result of the fault such as a poor connection, such the fault can be easily detected.
According to a third aspect of the present invention, a diagnosis portion detects a difference between the sensor signal before changing the current and the sensor signal after changing the current.
The diagnosis portion detects an occurrence of fault based on the difference in the sensor signal. Therefore, when the resistance is changed as a result of the fault such as a poor connection, such the fault can be easily detected.